The present invention relates to compositions and methods for their use in cleaning or etching of wafers used in the fabrication of semiconductor devices. More particularly, the invention relates to compositions for selectively removing low-k dielectric layers and/or photoresist layers on the wafers while controlling the rate of removal of the material, and methods using the compositions.
Many semiconductor processes require wafer measurements to be made from time to time for production-line quality control, product stability, and conformance to specifications. Generally, the processing results are evaluated after each significant step. Since many tests are destructive or will harm the product wafer, the measurements are typically performed on a test or xe2x80x9cdummyxe2x80x9d wafer, which are blank wafers that are mixed in with the product wafers. Dummy wafers are specially prepared to have similar characteristics as the product wafers. Typically, a dummy wafer is made of silicon, although other alternative materials have been described. Since silicon wafers are expensive, when the processing and testing is completed, the dummy wafers are usually cleaned, inspected, and reused. In the recovery or recycling of dummy wafers, it is important to provide a clean surface for subsequent processing. In the recycling of dummy wafers used in a masking photoresist process, it is important to completely remove blanket films of oxide and resist layers from the surface of the wafer down to the bare silicon.
Wet chemical processing is conventionally used for cleaning the surfaces of wafers. The cleaning is performed in order to produce a wafer having a silicon surface that is hydrophobic and repels water as the wafer is pulled out of the bath. The wafers are then rinsed in deionized water, dried in heated nitrogen gas (N2), and transferred to a processing operation, or stored.
Aqueous mixtures of hydrogen peroxide (H2O2) and sulfuric acid (H2SO4) are commonly used to strip or remove photoresist from a wafer surface after an etch processing step, or to remove a misaligned resist pattern from a wafer for re-imaging after development and inspection (xe2x80x9creworkxe2x80x9d). In such applications, it is desirable that the layer that is present under the resist, including, for example, silicon dioxide, remains intact on the wafer surface. In such cases, it is important to quickly remove the photoresist layer without attacking the underlying material. However, organic components in an underlying low-k dielectric layer, can alter the selectivity of the dielectric material to a resist layer.
In other applications, it is desirable to completely remove the oxide layer from a silicon surface. Hydrofluoric acid (HF) is conventionally used as a component of oxide etchants. HF solutions provide selective etch rates of about 100:1 or better for SiO2:Si, and effectively etch away the oxide but not the underlying silicon wafer material. Prevalent HF solutions include, for example, a 1:10 mixture of 49% hydrofluoric acid (HF) and water, and a dilute 1:6 aqueous mixture of HF and ammonium fluoride (NH4F). However, wet HF chemistries have difficulty removing low-k dielectrics such as a TMS-based low-k dielectric film, largely due to organic components in the oxide films. As a result, cleaning with an HF solution can leave the wafer with a gummy organic residue on the surface.
Therefore, it would be desirable to provide an improved composition for dissolution of low-k dielectric films from silicon-comprising surfaces. It would also be desirable to provide a cleaning composition that will completely remove low-k dielectric and photoresist components from the surface of a substrate such as a dummy wafer. It would also be desirable to provide a composition that can be modified to provide removal of a low-k dielectric material such as a TMS-based low-k dielectric film, from a wafer surface over a range of removal rates according to the application at hand.
The present invention provides a cleaning composition for use in wet cleaning or etching of a wafer surface, and a method of cleaning or etching wafer surfaces by applying the composition.
The cleaning composition is an aqueous solution that includes one or more effective amounts (v/v) of one or more fluorine-comprising compounds and one or more compatible acids to achieve the desired selectivity to resist and/or a low-k dielectric, and the desired rate of removal of the low-k material and/or the resist. A preferred composition according to the invention is an aqueous solution consisting essentially of the one or more fluorine-comprising compounds and the one or more acids.
The cleaning composition can be modified according to the invention to provide removal of a low-k dielectric material from the surface of a semiconductor wafer or other substrate over a range of removal rates according to the application at hand. In particular, the fluorine-comprising compound and the amount that is used, and also the amount of the acid component that is used, can be varied according to the invention to provide rapid removal of a low-k dielectric material and/or organic materials such as photoresist from the surface of the wafer at a rate of removal of greater than about 1000 angstroms per minute, or to provide controlled removal of either or both of those materials from the wafer surface at a slow to moderate rate of removal of about 50 to about 1000 angstroms per minute. The composition of the invention is useful in removing low-k dielectrics including, for example, trimethylsilane-and dimethylsilane-based low-k dielectrics.
For example, the composition can be formulated for use in a processing step such as a post-etch cleaning where it is desirable to remove low-k dielectric material at a slow and controlled rate so as to provide better process control. In that case, the cleaning composition is formulated with a relatively dilute concentration of the fluorine-comprising compound. By comparison, in a dummy recycling operation after a photoresist deposition or etching step, it is highly desirable to quickly and completely remove the low-k dielectric material and any photoresist or other organic components that may be present from the surface of the wafer, whereby the surface of the wafer is hydrophobic and water sheets off the surface of a wafer. In that case, a relatively high concentration of fluorine-comprising component is used in the composition. The composition can also be formulated to remove photoresist from a wafer, for example, after a mask operation, and leave the underlying low-k dielectric layer intact.
Suitable fluorine-comprising compounds are those that will speciate or ionize to provide fluoride ions at about pH 3 to 9. The fluorine-comprising compound can be an inorganic fluoride compound such as hydrofluoric acid (HF) and ammonium fluoride (NH4F), or an organic fluoride compound such as tetramethylammonium fluoride and hydrogen fluoride pyridiniun. Combined with the inorganic fluorine-comprising compound is an organic acid component that can be one or more of a monobasic, dibasic or tribasic organic acid, and is preferably citric acid, acetic acid, ascorbic acid, or mixtures thereof. An organic fluorine-comprising compound, when used, is combined with an inorganic acid such as sulfuric acid, among others. The acid component is included in an amount to control the pH of the composition at about 3 to about 9.
The invention also provides a method of cleaning a surface of a wafer. One embodiment of the method includes providing an aqueous cleaning composition that includes at least one fluorine-comprising compound and at least one acid component, and has a pH of about 3 to about 9; providing a wafer having a low-k dielectric layer and an overlying photoresist layer on at least one surface; and contacting the surface of the wafer having the low-k dielectric material and photoresist layers thereon with the cleaning solution under conditions effective to remove both of the low-k dielectric material and the photoresist layers substantially completely from the surface of the wafer, at a desired rate of removal whereby the resulting cleaned surface of the wafer is hydrophobic and water sheets off the surface of the wafer.
Another embodiment of the method includes providing an aqueous cleaning composition that includes at least one fluorine-comprising compound and at least one acid component and has a pH of about 3 to about 9; providing a wafer having a masked or unmasked low-k dielectric material on at least one surface; and contacting the surface of the wafer having the low-k dielectric material thereon with the cleaning solution under conditions effective to remove at least a portion of the low-k material at a desired rate of removal.
A further embodiment of the method includes providing an aqueous cleaning composition that includes at least one fluorine-comprising compound and at least one acid, and has a pH of about 3 to about 9; providing a wafer having a low-k dielectric layer and an overlying photoresist layer on at least one surface; and contacting the surface of the wafer having the low-k dielectric and photoresist layers thereon under conditions effective to remove the photoresist layer at a desired rate of removal while leaving the a low-k dielectric layer essentially intact.